In recent years, there has been a desire to build a society of sustainable development, and materials are being actively developed using recyclable resources also in the field of polymeric materials. Polylactic acid can be synthesized from plant-derived materials and exhibits mechanical strength similar to that of conventional plastics, and has been attracting attention as an alternative to petroleum-derived plastics. However, polylactic acid is poor in heat resistance and flexibility, which poses a major challenge for developing a wide range of applications.
As an example of polylactic acid having enhanced flexibility, disclosed is a polylactic acid resin composition in which 10 to 40 wt % of lactic acid or lactide or 10 to 60 wt % of a linear lactic acid oligomer or a lactide oligomer is added to a polylactic acid (Patent Document 1). However, when a low-molecular compound is contained in the resin, it is likely to cause an ooze out, i.e., bleed out, on the surface. Since the low-molecular compound is compatible with the noncrystalline portion of the polylactic acid, the mobility of the molecular chain of this portion is increased with the additive. Application of external energy allows the noncrystalline portion to gradually crystallize, diminishing the compatible portion, where the low-molecular compound is pushed out on the surface. On the other hand, when a linear lactic acid oligomer of a relatively high molecular weight is blended, it is hardly to cause a bleed-out, unlike using a low-molecular compound. However, the added lactic acid oligomer may inhibit plasticization because it is crystallizable per se.
As another example of polylactic acid having enhanced flexibility, disclosed is a polylactic acid resin composition to which a lactic acid derivative is added (Patent Document 2). However, also in this case, the additive is a low-molecular compound and it is likely to cause a bleed-out depending on the added amount.
As still another example of polylactic acid having enhanced flexibility, disclosed is a resin composition composed of a cyclic lactic acid oligomer and a polylactic acid (Patent Document 3). The combination of the oligomer and the polymer both derived from lactic acid can impart flexibility, thus maintaining the transparency and the biomass degree (the proportion of plant-derived materials). However, there are problems of insufficient enhancement of plasticization, low yield of the cyclic lactic acid oligomer, and difficulty in the control of the molecular weight of the cyclic oligomer.
In addition, modification by blend for other resins and fillers has been investigated to impart flexibility to polylactic acid. However, there are problems of; for example, impaired transparency of polylactic acid and decrease in biomass degree.
Meanwhile, to enhance the heat resistance of polylactic acid, for example, an inorganic filler, such as talc or mica, having heat resistance is added to polylactic acid. Specifically, adding an inorganic filler having a high heat resistance to polylactic acid results in effects of improvement in the mechanical properties of the polylactic acid and hardening of the polylactic acid. However, it is difficult to ensure a sufficient heat resistance in practice by adding an inorganic filler to polylactic acid alone, which causes another problem, i.e., increased specific gravity.
Although polylactic acid can affect a crystalline structure, it is a macromolecule which is hardly to crystallize. When polylactic acid is molded in the same manner as ordinary commodity-grade resins, the molded article will be noncrystalline (amorphous), which has a poor mechanical strength and is likely to exhibit thermal denaturation.
In this regard, polylactic acid can be crystallized by performing heat treatment during or after molding, thus enhancing the heat resistance of the molded article. However, the method for crystallization by heat treatment has a problem in practice due to a long time required for crystallization. It takes a considerable amount of time to complete the crystallization through a heat treatment of a molded article of polylactic acid in a mold.
When polylactic acid is crystallized alone without adding a material that serves as a crystal nucleus for polylactic acid (homogeneous nucleation), the size of crystal is as large as approximately a micron order due to a low frequency of spontaneous generation of crystal nucleus. Thus, the crystal of polylactic acid causes light scattering per se. Accordingly, turbidity and less transparency occur, deteriorating practical effectiveness.
To address such a problem with regard to a polymer that can affect a crystalline structure, that is, to facilitate the crystallization of a polymer that can affect a crystalline structure, addition of a crystal nucleation agent (nucleating agent) has been investigated. The “nucleating agent” refers to that which serves as a primary crystal nucleus for a crystallizable polymer to facilitate crystal growth of the crystallizable polymer. In a broad sense, the “nucleating agent” may refer to that which facilitates the crystallization of a crystallizable polymer, i.e., that which accelerates the rate of the crystallization of a polymer. By adding a nucleating agent to a resin, the crystal of polymer can be refined, resulting in effects such as improved rigidity and enhanced transparency, and it is also advantageous in shortening the molding cycle when the crystallization is performed during molding since the nucleating agent increases the rate (that is, reduces the time) of the entire crystallization.
Examples of nucleating agents that facilitate the crystallization of polylactic acid include addition of talc, layered clay compounds, dimethyl sulfoisophthalate metal salts, copper phthalocyanine, alloys with polycarbonate, and the like. However, any of such nucleating agents are not plant-derived additives through the use of a recyclable resource, which cause decrease in biomass degree and is problematic.
As for an additive which does not result in decrease in the biomass degree, disclosed is a block polymer of component A composed of poly-D-lactic acid, or a copolymeric resin of D-lactic acid and starch, with component B composed of a biodegradable resin having a melting point or a softening point lower than those of polylactic acid, which is blended into a polylactic acid (Patent Document 4). However, since this additive has a poor compatibility with polylactic acid, it may impair the stable compatibility in the resin composition, which is problematic and thus is required to add a compatibilizer, causing decrease in biomass degree which is also problematic.
Disclosed is that an amino acid such as tryptophan or phenylalanine is blended into a polylactic acid (Patent Document 5). However, such an additive has a poor compatibility with polylactic acid, and it is necessary to control the particle size distribution and the added amount, and also to add an agglomeration inhibitor to prevent agglomeration, which would cause decrease in biomass degree and is problematic.
Furthermore, prior nucleating agents fail to provide a sufficient crystallization facilitating effect and there is a problem that a sufficient degree of crystallization, in turn, heat resistance, cannot be obtained from the same molding cycle as that used on ordinary commodity-grade resins. The melting point of polylactic acid is 170° C., and it is believed that such a problem is overcome if crystallization proceeds sufficiently.
Discloses is a star-branched polyester polyol obtained by polymerizing lactide or lactic acid using a fat and oil as an initiator (Patent Document 6). However, this branched polymer is low crystallizable and does not demonstrate an effect to facilitate crystallization when added to polylactic acid.